An ideal cryogenic thermometer would be convenient to use, capable of measuring a wide range of temperatures near 0.degree. K., and preferably be insensitive to magnetic fields. Commonly used in the field are platinum and germanium resistance thermometers. These are convenient to use but most are sensitive to magnetic fields (Rubin et al., Cryogenics 22 491 (1982)). Carbon-glass thermometers, although less affected by magnetic fields, are useful only over a limited temperature range (Sample et al. Rev. Sci. Instrum. 53 1129 (1982)). Gas and capacitance thermometers, while unaffected by magnetic fields require frequent calibration (Rubin et al.).
Thin films of granulated, conducting metals dispersed in non-conducting matrices, prepared by radio frequency (R.F.) sputtering, have been considered for use as resistance elements in resistance thermometers. Thin films are attractive for this use because they have low heat capacities and thus adjust quickly to changes in temperature. It is to this class of material that the granular nickel in silica films of the instant invention belong. These and similar films were reviewed by Abeles et al. in Adv. Phys. 24 407 (1975).
Abeles et al. prepared and studied a number of films, some having compositions similar to but not the same as those of the instant invention. FIG. 25 of the Abeles reference reports the resistances of such nickel in silica films containing from 0.44 to 0.08 volume fraction of Ni (approximately 75 to 25 atomic percent as defined by x in the formula Ni.sub.x (SiO.sub.2).sub.(100-x)). Of these films, the closest in composition to those of the instant invention is the one containing 0.44 volume fraction (about 75 atomic percent) nickel. The resistance of this film was measured at temperatures as low as about 8.26.degree. K. (1/T.degree..sup.5 =0.35). The results shown in FIG. 25 of Abeles et al. were disappointing for the designer of a temperature-measuring resistance element because, as is also shown in FIG. 2 of the instant application, the resistance increases by a factor of about 10 million in going from about 100.degree. K. to about 10.degree. K. Below about 50.degree. K. the resistance would be said to be rapidly "approaching infinity". As is known it is difficult to measure very high resistances with accuracy. The results of Abeles et al. do not suggest that unexpectedly useful properties, as set out hereinafter, are to be found in films containing somewhat lesser concentrations of nickel at temperatures less than 50.degree. K.
A recent report (Yotsuya et al. in Appl. Phys. Lett 51 235 (1987) proposed reactively sputtered zirconium nitride in this use. Preliminary measurements by Unruh et al. as set out in Bull. Amer. Phys. Soc. Mar. Mtg. 1989 paper S18 9 indicate that this film is sensitive to magnetic fields at temperatures below 1.degree. K.